Galaxy Classification

Galaxies

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Galaxy Classification.

Galaxies are aggregates of gas, dust and millions (sometimes even billions or trillions) of stars held together by mutual gravitational forces. There are three categories of common galaxies and several other categories of less common ones, all classified by shape.

The three types of common galaxies.

Elliptical galaxies Elliptical galaxies, also known as Type E Galaxies, range in shape from E0, which is almost spherical, to E7, which is highly elongated. Elliptical galaxies contain little or no gas or dust, and are thought to consist almost entirely of old stars. While less conspicuous than spiral galaxies, because of their generally smaller size, they are actually more numerous.

E0	E3	E7
Spiral Galaxy. Spiral Galaxies, also known as Type S galaxies, are those with a central structure from which extend curving arms. They range from S0, which are fairly featureless and circular, through Sa, Sb, and Sc which are increasingly spread out with the nuclei getting increasingly fainter. Our galaxy, the milky way, is thought to be a type Sb. Spiral galaxies contain much gas and dust as well as both old and young stars.

S0	Sa	Sb	Sc
Barred Spiral Barred spiral galaxies , also known as Type SB galaxies, are a sub division of spiral galaxies and have a bar of stars and intersteller matter running through their nuclei. They range from SBa through SBb and SBc with spiral arms increasingly spread out and the nuclei getting progressively fainter.

SBa	SBb	SBc
Irregular Irregularly shaped galaxies, also called Type I galaxies, have no regular shape or have only a hint of one. The large Magellanic cloud, a companion galaxy to the milky way, is believed to be an irregular galaxy.

Irregular